1. Field of the Invention
The present invention relates to a printing apparatus to form images in accordance with image signals or image information obtainable from images on source documents. Specifically, the invention relates to a printing apparatus to which an ink jet recording method, thermal transfer method, or other recording methods are applicable. More particularly, it relates to a printing apparatus using an ink jet recording method capable of recording in high recording duties with the provision of many numbers of discharge nozzles.
Printing apparatuses of the present invention are those applicable as a textile printing apparatus to print on fabrics, a printing apparatus adapted for industrial use, or the like.
2. Related Background Art
There has been known an ink jet recording method whereby to record by discharging ink from discharge ports onto a recording medium and cause it to adhere to the medium. This ink jet recording method is widely used for comparatively small print output apparatuses for personal use. In a printing apparatus using such ink jet recording method, the temperature of printing head changes depending on the history of printing operations and recording duties. Then, as the temperature changes, its recording density is caused to vary, leading to unevenness created in recorded images. Particularly when the printing operation is carried out continuously or a high duty recording is performed, the temperature rise of the recording head becomes intense, making it difficult even to discharge ink in some cases. Also, in a printing apparatus capable of recording in colors with the provision of printing heads arranged respectively for each of plural colors, unevenness is created in each color used for recording, hence causing the ratio of color mixture to be varied when forming an image by mixing plural colors. It is inevitable that the color appearance varies in an image thus printed.
On the other hand, in a printing apparatus having a smaller number of nozzles to discharge ink, which prints at a comparatively slow speed, the heat generation of the printing head is also comparatively small. For such printing apparatus, the following proposals have been made in anticipation of solving the problems related to the temperature rise of printing head:
1) In U.S. Pat. No. 4,910,528, a technique is disclosed to suppress the excessive rise of temperature by detecting the temperature of printing head, thus controlling its printing speed accordingly so that the temperature of the printing head is adjusted.
2) In Japanese Patent Publication No. 3-4394, a technique is disclosed to suspend printing operation when the temperature of printing head exceeds a given temperature, and then, to resume the printing operation after waiting for the temperature to drop.
3) In Japanese Patent Publication No. 61-17670 and Japanese Patent Publication No. 62-41114, a technique is disclosed to control printing speeds by grasping printing duties in advance in accordance with recording data so that the temperature of the printing head is kept at a constant level.
4) In Japanese Patent Laid-Open Application Nos. 1-127361, 3-43254, and 4-47948, a technique is disclosed to stabilize the temperature of printing head in such a manner that while applying printing signals in an intensity good enough to generate thermal energy to the heaters of ink discharge nozzles that discharge ink, signals are applied to the nozzles that do not discharge ink in an intensity just to generate thermal energy but not strong enough to cause ink to be discharged, and that the resultant heat generation is made constant for the printing head: here, the arrangement of such means is well balanced with the arrangement of natural cooling.
5) In U.S. Pat. No. 4,929,963, a technique is disclosed to implement the cooling of a printing head by circulating ink itself in the interior of the printing head.
6) In EP Patent No. 573,062, a technique is disclosed to provide a mechanism to cause cooling water to pass a thermal transfer printing head, thus forcibly cooling such head by use of the cooling water.
In each of the proposals described above, however, the following problems are encountered:
In the paragraphs 1) to 3) described above, the temperature of the printing head is prevented from rising by the utilization of natural radiation. Thus the temperature of the printing head is not controlled, to be exact. As a result, if the recording duty is high, the printing speed is reduced, and also, there is a fear that the recording densities vary. Should a technique of the kind be applied to an ink jet textile printer or an ink jet printer for industrial use, the temperature rises immediately because such printers are provided with a large number of nozzles, and the recording duty is high, resulting in an enormous amount of heat generation. Therefore, such printer tends to suspends its operation frequently in order to allow its printing head to be cooled. Also, in this case, even if the printing speed is reduced, it is impossible to avoid the temperature rise of the printing head because of an enormous number of nozzles to be used. In any case, a printer of the kind makes it difficult to effectuate a continuous operation. Also, it is difficult to suppress the variation of recording densities due to the frequent changes of temperature while executing the printing operation.
In the technique described in the above paragraph 4), it is arranged that in addition to the energy for discharging ink, energy for generating heat is given even when no ink is discharged in order to make the heat generation of the printing head constant, and then, on the assumption that the natural radiation is constant, an arrangement is made to make the heat generation of the printing head well balanced with the natural radiation. In this way, the temperature of the printing head is stabilized at a constant level.
With this arrangement, however, the total amount of heat generation of the printing head becomes enormous, resulting in a more intensive rise of temperature than that of the examples referred to in the above paragraphs 1) to 3). Thus, the continuous operation becomes more difficult. In order to perform the operation continuously, the nozzle numbers should be smaller, and also, the printing speed is made slower. In addition, the heat generation of the printing head should be smaller so as to be able to anticipate a sufficient natural radiation. Even under such conditions, the balancing point with the natural radiation varies as the inner temperature of the apparatus rises in operation. Further, the balancing point of the head temperature varies eventually as the outside temperature changes.
In the method described in the above paragraph 5), ink is circulated between the printing head and ink tank to cool the printing head. However, the method referred to in that paragraph does not provide any mechanism to adjust the temperature of ink. Therefore, the temperature of circulating ink rises without any control as a printing operation is carried on continuously, leading to the creation of a gradual variation of recording densities after all. Further, before this problem arises, if this method is adapted for a printing head provided with many numbers of nozzles for use of high duty recording, that is, if it is used for cooling a printing head that may generate an enormous heat generation, there is a need for executing the circulation of ink in an amount sufficient enough to effectuate the intended cooling. Then, due to the pressure exerted by such circulating ink, the menisci of ink in the nozzles are broken to allow ink to leak from the nozzles or air to enter the nozzles to mix with ink. In such a case, there is a fear that the printing function itself is even liable to malfunction.
The methods described in the above paragraphs 1) to 5) are all applicable to printing apparatuses having a smaller number of nozzles for the execution of low duty recording, that is, those of a small type for office use, for which it suffices to utilize the natural radiation appropriately, and the variation of recording densities and unevenness in recorded images do not present such a serious problem as in the case of industrial use. For printing apparatuses for industrial use that require high recording duties, demands are so strict with respect to the variation of recording densities and unevenness in recorded images that any one of the methods described above is not adoptable.
Also, the method described in the above paragraph 6) discloses a mechanism using cooling water to cool a specific printing head of a thermal transfer type, but there is no consideration given to any application at all to printing apparatuses of various types including those for industrial use.
As described above, even with the adoption of any one of the conventional methods, it is impossible to materialize an ink jet recording apparatus for industrial use, which is capable of printing in high recording duties with a good color reproducibility of prints without any variation of recording densities and unevenness in the recorded images.
Now, the ink jet technologies have reached the stage almost equal to the technical level of the offset printing in such areas as the image processing technique using error diffusion methods or the like, the graduation expression technique obtainable by the combination of such image processing technique and multiple composition printing; and in the image expression capability obtainable by the application of ink jet recording technique, which uses the wide color reproducibility by means of multicolor printing in six to ten color process beyond the conventional four prime color process.
Also, with such advanced technologies, a serial printing method is now adoptable for printing a screen of wider and longer size, and it is anticipated that the range of application of this method is expandable to the industrial fields such as ink jet textile printing, printing of large image screens, fabrication of color filters for use of display, and some others. However, there are problems yet to be solved as described above in this respect.
The ink jet technologies provide readiness as an output means for the computer publishing that uses a computer as its host device. Also, when applying the technologies, there is no need for the provision of steps to prepare impressions. It is not required to mix ink to obtain colors as desired, either, among other significant advantages. However, due to the problems described above, the application of these technologies has not been sufficiently developed as yet with respect to its use in the industrial fields.